Remove stale ignore_column entries from CD models

What does this MR do and why?

Remove overdue ignore_column entries from CD models. Each one tracked a column that was already removed in milestone 19.2 (released), so the ignores are leftovers that should have been cleaned up in 19.3. This is a model-only change: the columns are already gone from the schema, so there is no migration and no db/structure.sql change.

An ignore_column is only safe to remove once its column has been dropped in a released milestone. All entries below were verified against db/structure.sql (column absent) and their drop migrations (all milestone 19.2).

Entries removed

Model	ignore_column removed	Column dropped by (19.2)
cd/artifact_source.rb	:project_id, :source_type	remove_cd_artifact_sources_legacy_columns
cd/rollout.rb	:environment_id	remove_environment_id_from_cd_rollouts
cd/version_set.rb	:environment_id	remove_environment_id_from_cd_version_sets
cd/environment.rb	:cluster_agent_id, :platform_type, :region	remove_cd_environments_legacy_columns
cd/deployment.rb	:rollout_id, :version_set_entry_id	remove_cd_deployments_legacy_columns
cd/rollout_transition.rb	:principal_type	cleanup_rename_principal_type_to_principal_on_cd_rollout_transitions
cd/deployment_transition.rb	:principal_type	cleanup_rename_principal_type_to_principal_on_cd_deployment_transitions

(principal_type was a concurrent column rename to principal; the cleanup step that dropped the old column shipped in 19.2, which is why it had an ignore_column.)

Kept intentionally

• :group_id (all CD models) — these columns are being dropped now via https://gitlab.com/gitlab-org/gitlab/-/work_items/602934; their ignore_column entries are removed in a follow-up next milestone.
• :principal_id (rollout_transition, deployment_transition) — the column is still present (remove_with: '19.4'), so its ignore stays until the column is dropped and that drop is released.

Why it's safe

• Columns are already absent from db/structure.sql.
• All drops are milestone 19.2 (released long before 19.4), so removing the ignores now cannot cause Rails to select a not-yet-dropped column during a deploy.
• No migration, no schema change; purely removing dead model annotations.

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/627529

MR acceptance checklist

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